Each year tens of millions of gallons and more of contaminated aqueous solutions are discharged as industrial wastes. These solutions may contain a variety of contaminants, including ionic selenium species, sulfate ions and ionic heavy metal species, substances which are objectionable from an environmental standpoint. Typical examples of industries which generate and discharge waste solutions containing contaminants such as those enumerated above include the chemical, metal processing and mining industries.
Due to the real as well as the potential adverse impacts that such contaminants as sulfate do and can have on man and environment, both federal and state agencies have promulgated and put into force numerous regulations establishing maximum concentration levels for these contaminants in industrial waste solutions discharged into our groundwater systems. As an example of such regulations, the current allowable maximum concentration levels for sulfate ions in mine discharge waters permitted by the State of New Mexico is 600 milligrams of sulfate per liter of the discharged water. Furthermore, this State also has established maximum concentration levels for such metals as selenium and heavy metals such as uranium and molybdenum of 0.05, 5.0 and 1.0 milligrams of said metals, respectively, per liter of the discharged water. Other state agencies have established similar regulations, although the maximum concentration levels permitted may vary from state to state.
In order to comply with these regulations, a number of processes have been used or are now undergoing testing and development to effect removal of various contaminants. Such processes include gravity sedimentation, flotation, filtration, ion exchange, activated adsorption, reverse osmosis, electrodialysis, distillation and chemical precipitation. However, many of these processes are not ideally suited to treat large volumes of water or are excessively expensive to install and operate in comparison with their overall efficiency and effectiveness.
In addition to the above processes, various biological processes are known for removing contaminants from waste water solutions, including the removal of sulfate and heavy metals. In such processes sulfate reducing bacteria are employed to reduce the sulfate to hydrogen sulfide which in turn reacts with the heavy metals to form water insoluble heavy metal sulfide which precipitate out of solution. Typical examples of such known biological processes for removal of sulfate and heavy metal contaminants from aqueous processing waste streams or solutions include U.S. Pat. No. 4,354,937 and U.S. Pat. No. 4,108,722.
Drawbacks to the use of biological processes, however, also are well known. These include, to illustrate but a few, the production of metabolic wastes which in and of themselves represent real or potential pollution problems and the formation of fine crystalline sulfide precipitates the removal of which, by sedimentation or filtration, is extremely difficult.
Furthermore, biological processes such as those enumerated above are not known to be capable of removing ionic selenium species which often are present in combination with sulfate and heavy metal contaminants in many aqueous processing waste streams. This is particularly true of aqueous waste streams such as discharge waters from uranium mining and processing operations.